Stannous salts are commonly known for their efficacy in oral healthcare. For example stannous fluoride has a long history of use in oral care products for protection against caries, pathogenic bacteria, plaque, gingivitis, dentine hypersensitivity and oral malodor. However there are some disadvantages associated with the use of stannous salts in oral care products, such as chemical instability, astringent taste and tooth discoloration as a result of build-up of extrinsic stannous stain. Accordingly oral care dentifrices containing stannous salts, such as stannous fluoride, have been formulated as dual phase compositions, in which the stannous salt is stabilized in a separate phase from the rest of the composition. The stannous is stabilized without increasing the negative aesthetics associated with stannous and ingredients commonly used to stabilize stannous. However dual-compartmented packages are typically considerably more expensive to manufacture than the conventional laminate tubes that have been used for many years to contain and dispense dentifrice products. Dual-phase products are also less convenient than single phase compositions in terms of ease of use by consumers.
Sodium tripolyphosphate is well-known both as an effective anticalculus (anti-tartar) agent and as an anti-stain agent. However chemical binding of stannous using pyrophosphates, diphosphonates or tripolyphosphates to prevent stain formation also produces unacceptable losses in therapeutic potential, according to U.S. Patent Application Publication No. 2005/0112070 A1). This patent application favours the use of alternative polyphosphates, namely those having an average chain length greater than four, in stannous-containing dentifrice systems.
Other attempts to provide efficacious dentifrice compositions comprising a stannous salt and polyphsophates are described in PCT patent applications WO2007/062365 and WO2011/053291.
WO2007/062365, (“Stannous Salt and Sodium Tripolyphosphate Oral Care Compositions and Methods”) describes oral care compositions comprising; (a) about 0.01 to about 5% stannous salt; (b) about 0.01 to about 15% sodium tripolyphosphate; (c) less than about 10% by weight water; and (d) about 0.1 to about 15% by weight of a methylvinyl ether-maleic anhydride copolymer; wherein the composition comprises less than about 1% of stannous tripolyphosphate ionic intermediates.
WO2011/053291, (“Dentifrice Comprising Stannous Fluoride Plus Zinc Citrate and Low Levels of Water”) describes dentifrice compositions having a low water phase comprising effective amounts of polyphosphate and ionic active ingredients. More specifically WO2011/053291 describes compositions comprising in a single phase: an orally acceptable vehicle; a source of fluoride ions; a source of stannous ions; a source of zinc ions; and at least one polyphosphate salt selected from the group consisting of inorganic polyphosphate salts which have equal to or less than three phosphorous atoms and wherein the dentifrice composition has a total water content of less than about 10% based on the weight of the composition.
According to WO2011/053291, the compositions therein may further comprise an aqueous buffer system e.g. a citrate buffer system, for the source of stannous ions. Example 1 therein discloses Formulas A, B, C and D comprising inter alia a source of zinc ions (zinc citrate); stannous fluoride and sodium tripolyphosphate (STP). Formulas A and B further comprise a citric acid buffer system whereas Formulas C and D, do not. When Formulas A and C were tested in an accelerated aging study to determine the stability of the stannous salt, Formula A was reported as showing only a small reduction (about 7%) in stannous availability, whereas Formula C showed a large reduction in stannous tin availability (about 33%) and so a formula such as Formula C, would not be stable throughout the normal shelf life of a dentifrice product (typically 12-24 months under ambient temperature).
Whilst dentifrice compositions comprising actives such as stannous salts and polyphosphates have been described in the prior art, it remains challenging to provide these actives in a stable single phase composition. Accordingly it is the aim of the present invention to provide such a composition and in particular which provides good long term storage stability, consistent with the requirements of a commercial dentifrice product.